Effectiveness of secondary tabs for supersonic mixing

The effect of vortex generators, in the form of small tabs projecting into the flow at the nozzle exit, aided by secondary tabs on either side, on the mixing characteristics of an axisymmetric jet at Mach number 1.7 is investigated. Experimental studies on the basic features of the jet from a nozzle with secondary tabs are conducted to assess the free jet characteristics as well as the momentum and thermal mixing behavior. The secondary tabs were found to increase the jet spread and distort the jet cross-section and were found to cause substantial enhancement of mixing of supersonic jets. Jet structure is observed using flow-visualization techniques. LLS images are employed to obtain cross-sectional views of the jet with the introduction of secondary tabs. The ability of secondary tabs to eliminate the screech noise of the supersonic jet is also observed. Received: 3 February 2000/Accepted: 8 February 2001     

Characteristics of Sonic Jets with Tabs

The results of an experimental investigation on the effect of a vortex generator in the form of a mechanical tab placed at the nozzle exit on the evolution of jet and its decay are reported in this paper. Jets from a sonic nozzle with and without tabs operated at nozzle pressure ratios from 2 to 7 were studied. Tabs with two combinations of length-to-width ratio were investigated by keeping the blockage area constant. The tabs offered a blockage of 10.18% of the nozzle exit area. The centerline pitot pressure decay shows that for the tabbed jet a maximum core reduction of about 75% can be achieved at a nozzle pressure ratio (NPR) 7 compared to an uncontrolled jet. The shadowgraph pictures show that the tabs drastically weaken the shock structure in the jet core and disperse the supersonic zone of the flow making them occupy a greater zone of the flow field compared to the plain nozzle. This causes the waves to become weaker and the jet to spread faster. The tabs are found to shed counter-rotating vortices all along the edges, resulting in enhanced mixing. Isobaric contours reveal that the streamwise vortices cause an inward indentation of the entrained mass into the jet core and an outward ejection of core flow. To understand the distortion introduced by tabs on the jet cross-section and its growth leading to bifurcation of the jet, a surface coating visualization method was developed and employed.     

Simulation of shear layers interaction and unsteady evolution under different double backward-facing steps

High-order accurate schemes are employed to numerically simulate the interaction of a supersonic jet and a co-directional supersonic inflow. A double backward-facing step model is proposed to investigate the interaction between the jet shear layer and the supersonic inflow shear layer. It is found that due to the interaction of the shear layer, a secondary jet is injected into the recirculation zone at the intersection of the two shear layers. The secondary jet produced by the interaction of the two shear layers has a periodicity because of shear layers interaction. The distinction in the shape of double backward-facing steps will induce changes in the period of the secondary jet. The analysis and discussion of the periodicity of the secondary jet are mainly focused in this letter.     

超声速钝体逆向喷流减阻的数值模拟研究

为研究逆向喷流技术对超声速钝体减阻的影响,采用标准k-ε湍流模型,通过求解二维Navier-Stokes方程对超声速球头体逆向冷喷流流场进行了数值模拟,并着重分析了喷口总压、喷口尺寸对流场模态和减阻效果的影响。计算结果显示:随着喷流总压的变化,流场可出现两种流动模态,即长射流穿透模态和短射流穿透模态;喷流能使球头体受到的阻力明显减小;存在最大减阻临界喷流总压值(在所研究参数范围内最大减阻可达51.1%);在其它喷流物理参数不变时,随着喷口尺寸的增大,同一流动模态下的减阻效果下降。本文的研究对超声速钝体减阻技术在工程上的应用具有一定的参考价值。     

Effect of Condensation on the Length of Strongly Underexpanded Jets Exhausting Into a Rarefied Submerged Space

Exhaustion of supersonic argon and nitrogen jets through sonic and supersonic nozzles into a rarefied submerged space at high stagnation pressures is studied experimentally. The shapes and lengths of the jets are visualized by means of detecting radiation excited in the considered flow by an electron beam. Dependences of the geometric parameters of the jets on exhaustion and clusterization conditions at low Reynolds numbers based on the reference length of the jet are obtained. It is found that the coefficient of proportionality between the length of the first “barrel” of the supersonic jet and the degree of jet expansion increases with an increase in the stagnation pressure. Empirical dependences of the proportionality coefficient on the size of clusters formed in supersonic flows are derived for the first time.     

On the mechanism of self-oscillations of a supersonic radial jet exhausting into an ambient space

Results of an experimental study and numerical simulation of self-oscillations of a supersonic radial jet exhausting from a plane radial nozzle into an ambient space are reported. It is demonstrated that flexural oscillations develop in the jet, leading to its destruction. Feedback ensured by acoustic waves in the gas surrounding the supersonic jet is found to play a key role in the emergence of self-oscillations.     

AN ANALYSIS ON COAXIAL JET FLOWS USING DIFFERENT DECOMPOSITION TECHNIQUES

In this study, the Fourier transform, wavelet transform and turbulence filter methods have been applied to coaxial jet flows for various downstream positions in the inner and outer mixing regions. The data were obtained from cross-wire measurements, so axial and radial velocity components were acquired. Attention is focused on the characteristics of these signals in the initial region of the jet-flow field. The effects of some basic mechanisms of the vorticity dynamics on the velocity time histories, and on different statistical and spectral quantities, are studied.     

Effect of sound on a supersonic jet

It is known that under the influence of sound from an external source or the sound emitted by the supersonic jet itself at discrete frequencies in nonoptimal flow regimes the supersonic jet expands more rapidly and its range is reduced [1, 2], However, the mechanism of action of the sound on the supersonic jet has not been adequately investigated and, in particular, no one has determined the intensity of the external source capable of producing a marked change in the gas dynamic parameters of the jet, its characteristics or how the interaction process develops. These questions are examined below. By means of shadow photography with a pulsed light source it is shown that a significant change in the gas dynamic characteristics of the supersonic jet can be achieved by directing at its base along the normal to the jet boundary sound with an intensity corresponding to 0.1–0.2% of the total pressure in the jet. The appearance of large-scale disturbances on the irradiated side of jet and the directional emission of sound by the jet at the frequency of the external source are noted.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 170–174, November–December, 1989.The author is grateful to A. A. Kochetkov for assisting with the work.     

Effect of Spherical Recesses on the Characteristics of Coanda Flow

Sub- and supersonic flows past curvilinear surfaces with spherical recesses are investigated. The Coanda flow was created by a jet flowing out from a plane convergent nozzle into a submerged space along the tangent to a circular cylinder. The forces exerted on the cylinder and the total and static pressure profiles in Coanda jet cross-sections were measured. It is shown that the spherical recesses increase the friction drag at both sub- and supersonic velocities.     

Flow and acoustic features of a supersonic tapered nozzle

The acoustic and flow characteristics of a supersonic tapered jet were measured for free and shrouded flow configurations. Measurements were performed for a full range of pressure ratios including over- and underexpanded and design conditions. The supersonic tapered jet is issued from a converging-diverging nozzle with a 31 rectangular slotted throat and a conical diverging section leading to a circular exit. The jet was compared to circular and rectangular supersonic jets operating at identical conditions. The distinct feature of the jet is the absence of screech tones in the entire range of operation. Its near-field pressure fluctuations have a wide band spectrum in the entire range of measurements, for Mach numbers of 1 to 2.5, for over- and underexpanded conditions. The free jet's spreading rate is nearly constant and similar to the rectangular jet, and in a shroud, the pressure drop it is inducing is linearly proportional to the primary jet Mach number. This behavior persisted in high adverse pressure gradients at overexpanded conditions, and with nozzle divergence angles of up to 35°, no inside flow separation was observed.     

UV-mediated coalescence and mixing of inkjet printed drops

In the present study, the characteristics of supersonic rectangular microjets are investigated experimentally using molecular tagging velocimetry. The jets are discharged from a convergent–divergent rectangular nozzle whose exit height is 500 μm. The jet Mach number is set to 2.0 for all tested jets, and the Reynolds number Re is altered from 154 to 5,560 by changing the stagnation pressure. The experimental results reveal that jet velocity decays principally due to abrupt jet spreading caused by jet instability for relatively high Reynolds numbers (Re > ~450). The results also reveal that the jet rapidly decelerates to a subsonic speed near the nozzle exit for a low Reynolds number (Re = 154), although the jet does not spread abruptly; i.e., a transition in velocity decay processes occurs as the Reynolds number decreases. A supersonic core length is estimated from the streamwise distribution of the centerline velocity, and the length is then normalized by the nozzle exit height and plotted against the Reynolds number. As a result, it is found that the normalized supersonic core length attains a maximum value at a certain Reynolds number near which the transition in the velocity decay process occurs.     

Experimental Investigation of Nozzle Exit Reflector Effect on Supersonic Jet

The present study describes an experimental work to investigate the effect of a nozzle exit reflector on a supersonic jet that is discharged from a convergent–divergent nozzle with a design Mach number of 2.0. An annular reflector is installed at the nozzle exit and its diameter is varied. A high-quality spark schlieren optical system is used to visualize detailed jet structures with and without the reflector. Impact pressure measurement using a pitot probe is also carried out to quantify the reflector’s effect on the supersonic jet which is in the range from an over-expanded to a moderately under-expanded state. The results obtained show that for over-expanded jets, the reflector substantially increases the jet spreading rate and reduces the supersonic length of the jet, compared with moderately under-expanded jets. The reflector’s effect appears more significant in imperfectly expanded jets that have strong shock cell structures, but is negligible in correctly expanded jet.     

金属射流发散性理论分析及数值模拟

分析经典射流理论和相关文献,给出了在汇聚点坐标系中金属Cu对碰形成射流的汇聚射流区、无射流激波封锁区、无射流强度封锁区和形成发散射流区域。对金属超声速射流形成的发散性问题进行了理论分析,诠释了Walker基于实验提出射流发散理论,证明了金属射流超声速部分可形成发散射流也可形成汇聚射流,且超过1.23倍体声速的金属射流必定是发散的。最后,应用自编的欧拉计算程序MEPH对金属Cu以不同速度、倾角对碰射流形成过程进行数值模拟,得到了分叉射流、空洞射流和密实稀疏射流等的典型射流发散模式图像,印证了理论分析的结果。     

激励射流在超声速流场中的混合的特性研究

Hartmann-Sprenger（H-S）管处于吞吐模式下可以产生高频高幅度的气流振荡。本文将H-S管集成到传统射流装置中形成激励射流,研究其在超声速流场中的混合特性。本文设计了三种频率的CO2激励射流,将其横向注入马赫数2.5的均匀超声速来流当中。采用平面激光散射技术对其进行瞬态可视化成像,利用组分采样、测量总压进行时均分析。结果表明:H-S激励射流可以有效提高射流穿透深度,形成较多大尺度结构,从而获得较好的混合效果。关键词: 激励射流 超声速混合 穿透深度 大尺度结构      

Flow in a viscous jet escaping through a supersonic nozzle into a semi-infinite ambient space

The problem of an axisymmetric gas flow in a supersonic nozzle and in the jet escaping from the nozzle to a quiescent gas is solved within the framework of Navier-Stokes equations. The calculated pressure distribution is compared with that measured in the jet by a Pitot tube. The influence of the jet pressure ratio, Reynolds number, and half-angle of the supersonic part of the nozzle on nozzle flow and jet flow parameters is studied. It is shown that the distributions of gas-dynamic parameters at the nozzle exit are nonuniform, which affects the jet flow. The flow pattern for an overexpanded jet shows that jet formation begins inside the nozzle because of boundary-layer displacement from the nozzle walls. This result cannot be obtained with the inviscid formulation of the problem.     

轻型火箭发射噪声场的分布特性

为了解小火箭发射噪声特性及其在喷口外围的声压场分布规律,针对燃气射流产生噪声问题进行了实验研究和数值计算。讨论了超声速射流噪声的3个主要成分(湍流混合噪声、啸音和宽带激波相关噪声)及相关特点,指出它们产生的根本原因是湍流射流的速度扰动。通过分析不同实验测点的射流噪声声压级峰值,得到了燃气射流噪声在轴向和径向上的分布规律,即随着离喷口距离的增大,轴向噪声的衰减程度大于径向。在实验基础上,利用大涡模拟与FW-H(Ffowcs Williams-Hawkings)声学比拟相结合的方法对燃气射流噪声的声学特性进行计算。结果表明,此方法获得的计算结果与实验结果吻合较好,可为进一步研究射流噪声控制提供参考。

     

Flow structure at the initial section of a supersonic jet exhausting from a nozzle with chevrons

The spatial structure of the flow in a supersonic underexpanded jet exhausting from a convergent nozzle with vortex generators (chevrons) at the exit is experimentally studied. Exhaustion of a supersonic underexpanded jet from a nozzle with chevrons at the nozzle exit is numerically simulated with the use of the Fluent commercial software package. The experimental and numerical data are demonstrated to be in reasonable agreement. The influence of chevrons on the process of gas mixing is estimated.     

Features of a Turbulent Jet at High Supersonic Velocities

The results of modeling a turbulent supersonic jet at M = 5 using large-eddy simulation (LES) are presented. The structural features of turbulence formed in this flow are analyzed. The possibilities of the large-eddy simulation method and the complexities of simulation of the compressibility effects in jet flows at high Mach numbers are considered. Such features of the supersonic jet as the local turbulent shocklets and Mach waves are reproduced numerically. It is shown that in the neighborhood of the jet the trajectories of ejection flow are located along the front of Mach waves. Anisotropic turbulent structures whose longitudinal scale is greater than the transverse scale by an order of magnitude are revealed in the jet. An estimate of the baroclinic effects shows their weak influence on the vorticity generation in the jet flow considered.     

Mechanism of self-oscillations in a supersonic jet impact onto an obstacle 1. Obstacle with a spike

Results of numerical simulations and experimental investigations of self-oscillations arising in the case of impingement of an overexpanded or underexpanded jet onto an obstacle with a spike are reported. The mechanisms of the emergence and maintaining of self-oscillations for overexpanded and underexpanded jets are elucidated. It is demonstrated that self-oscillations are caused by disturbances in a supersonic jet, which induce mass transfer between the supersonic flow and the region between the shock wave and the obstacle. The feedback is ensured by acoustic waves generated by the radial jet on the obstacle. These waves propagate in the gas surrounding the jet, impinge onto the nozzle exit, and initiate disturbances of the supersonic jet parameters. In the overexpanded jet, these disturbances penetrate into the jet core, where they are amplified in oblique shock waves.     

Experimental study on the condensation of supersonic steam jet submerged in quiescent subcooled water: Steam plume shape and heat transfer

The condensation of supersonic steam jet submerged in the quiescent subcooled water was investigated experimentally. The results indicated that the shape of steam plume was controlled by the steam exit pressure and water temperature. Six different shapes of steam plume were observed under the present test conditions. Their distribution as a function of the steam exit pressures and water temperatures was given. As the steam mass velocity and water temperature increase, the measured maximum expansion ratio and dimensionless penetration length of steam plume were in the ranges of 1.08–1.95 and 3.05–13.15, respectively. The average heat transfer coefficient of supersonic steam jet condensation was found to be in the range of 0.63–3.44 MW/m²K. An analytical model of steam plume was found and the correlations to predict the maximum expansion ratio, dimensionless penetration length and average heat transfer coefficient were also investigated.